This invention relates to high power (nonlinear) testing of microwave transistors (DUT) in the frequency and time domain [1]. The electrical signals injected into the input of the DUT and extracted from the output can be measured using sampling devices, such as signal couplers [6]. At high power the (nonlinear) DUT is saturating and deforming the sinusoidal input signal. As a result part of the power is contained in harmonic frequency components. The DUT performance can only be optimized when all harmonic frequency components are impedance-matched properly. This requires independent harmonic tuning, mainly but not exclusively, at the DUT output.
Load pull is the method by which the load impedance presented to the DUT at a given frequency is changed systematically and the DUT performance is registered, with the objective to find an optimum depending on the overall design objectives [1]. This may be maximum power, efficiency, linearity or else. The same is valid for the source side of the DUT. Passive (slide screw) tuners are used to emulate the various impedances presented to the DUT [3]. Insertion loss in the signal path between the DUT and the tuner reduce the “tuning range” of the tuner and it often happens that the DUT cannot be matched properly, if its internal impedance cannot be reached by the tuner (reduced by the insertion losses of the transmission section between tuner and DUT). In these cases “active load pull systems” can be used [10].
Active load pull systems use amplifiers in an “open” or “closed” loop configuration to recover and re-inject part of the outgoing power into the DUT in order to generate a “virtual” load. Depending on the gain and power of the amplifiers in the loops the returning power can be equal or even higher than the outgoing power, so the virtual reflection factor δ, defined as |Γ|2=Pr/Pi, where Pr is the power reflected by the load and Pi the power delivered by the DUT, presented to the DUT can be equal or even higher than 1. The subject of this invention is to introduce a component which allows controlling the amplitude and phase of the signals returned to the DUT by the “active” loop.
In the case of open loop active load pull systems there are, in principle, two ways for injecting signal power at controlled amplitude and phase into the DUT. One is by using external synchronized signal sources at the designated (harmonic) frequencies and the other is to use closed loops and control amplitude and phase individually [11]. This second option requires a “frequency selective amplitude and phase controller”. Such an apparatus is presented here.
A device capable of controlling the amplitude and phase of signals in the microwave frequency range individually for a number of frequencies is not known. What is known are commercially available variable attenuators (FIG. 1) and phase shifters (FIG. 2). Whether those components are manually or remotely controlled is irrelevant. What is important is that they are “wideband”; this means the amplitude and phase of two signals with different frequencies going through those components cannot be controlled independently. This makes those components useless for the present application.
The present invention describes an apparatus that can do that; it can adjust, independently, the amplitude and phase of a number of signals at different frequencies passing through it.